The drive force required for the motion of floating vessels is provided by propellers equipped with various driving mechanisms. In shaft drive vessels, shafts are generally communicated to the inboard motor so as to make an angle of about 10° downward from the water surface. While propeller performance in shaft drive vessels is high up to speed levels of 30-40 knots of the vessel, it decreases in speed levels of 50-60 knots due to the increasing need for motor power. Especially at high speeds, surface drive propellers may be preferred for increasing propeller performance. In such propellers, the propeller shaft extends linearly backward from the boat transom, some of the propeller blades remaining under water level, while some remaining on the water surface. The vessel is thus provided with drive force by a propeller operated by a lower motor power since the propeller device and some of the blades are not subject to water resistance.
It has been aimed to increase propeller performance by partly moving some known surface drive propellers in vertical direction. For example, U.S. Pat. No. 5,279,509 discloses a surface drive propeller device which can perform up and down swinging movement with the steering mechanism thereof. Swing of the propeller is essentially provided by changing the rotational axis of the shaft, which extends from the boat transom, by means of a universal joint. U.S. Pat. No. 5,279,509 cannot provide the efficiency provided by a conventional shaft drive propeller at low boat speeds since the propeller blades still remain above the water level during maximum downward inclination of the shaft. Moreover, although propeller blades could totally remain under water level, it is not possible, by using a power transmission means based on a sole universal joint, to achieve transmission of the motor torque to the propeller in order for the propeller to provide sufficient drive force.
Apart from shaft drive and surface drive propellers, pod-drive and Z-drive propellers are also used in the art, but these are far from achieving optimum propeller performance according to changing cruise conditions.
Thus, there exists a need for a propeller device which serves as a shaft drive propeller at relatively lower speeds of the vessel or during the compensation of starting inertia, or in conditions where water resistance on the hull is relatively lower, while acting like a surface drive propeller at relatively higher boat speeds.